What Contribution Could Industrial Symbiosis Make to Mitigating Industrial Greenhouse Gas (GHG) Emissions in Bulk Material Production?

In industrial symbiosis, byproducts and wastes are used to substitute other process inputs, with the goal of reducing the environmental impact of production. Potentially, such symbiosis could reduce greenhouse gas emissions; although there exists literature exploring this at specific industrial sites, there has not yet been a quantitative global assessment of the potential toward climate mitigation by industrial symbiosis in bulk material production of steel, cement, paper, and aluminum. A model based on physical production recipes is developed to estimate global mass flows for production of these materials with increasing levels of symbiosis. The results suggest that even with major changes to byproduct utilization in cement production, the emission reduction potential is low (7% of the total bulk material system emissions) and will decline as coal-fired electricity generation and blast furnace steel production are phased out. Introducing new technologies for heat recovery allows a greater potential reduction in emissions (up to 18%), but the required infrastructure and technologies have not yet been deployed at scale. Therefore, further industrial symbiosis is unlikely to make a significant contribution to GHG emission mitigation in bulk material production.


Material flow charts
The process flow charts in the following sections represent the stylised production routes for assessing the industrial symbiosis potential. Each process is represented with a box and the main output flow is visualised with a circle. The processes which only operate in the two scenarios of industrial symbiosis are highlighted with colours: orange for processes operating in both scenarios and green for those added only in scenario B (full symbiosis).

Steel production
The process route for the blast furnace steel production used in the model is depicted in Figure S 1.

S5
The process route for the electric arc furnace steel production used in the model is depicted in Figure  S 2.

Cement production
The process route for the integrated cement production used in the model is depicted in Figure S 3.

Aluminium production
The process route for the aluminium production used in the model is depicted in Figure S 4.

Pulp and paper production
The process route for the primary paper production used in the model are summarised in Figure

Process recipes
The production recipes of the processes used in the model are summarised in this section in tables with material flows and further information. The production recipes were used for the production coefficient matrix for the production system.
The production processes were divided in four categories: 1. Main industrial processes: the main bulk material production processes and the processes that produce the inputs of them. 2. Energy processes for providing heat and electricity: processes for steam and electricity generation. 3. Industrial symbiosis processes for preparing materials for utilisation and combustion: processes that turn one by-product into a usable input of another process. 4. Processes for providing resources from nature and utilising by-products from industry: extraction processes (e.g. mining) and processes in other industries (e.g. chemicals), that provide the inputs needed for the major bulk material production processes.
During the data and literature review, the production recipes were collected for all major processes and sub-processes. The main industrial processes consist of several sub-processes which are summarised in the first section of this supplementary information. The sub-processes provide all inputs required for producing the final material along the process chain. The key parameters are summarised in Table S 1.

S12
The global material production for primary and secondary production in 2017 is used in the model for the global material demand ( "). It is summarised in

Main industrial production processes
The following sections contain the production recipes data tables used in the global production model. Table S 4 summarises the processes and sources used for the production recipes. Paper Process-level data from the BAT documents, two peer-reviewed analyses of the material flows in paper production systems and emissions data from LCI database Ecoinvent.
For steel mass flows, the global average steel resource data for BF and EAF steel production from Gonzalez Hernandez et al. 6  For aluminium production processes, mass flow information was used from Balomenos et al. (2017) and temperature information was taken from Nowicki and Gosselin (2012). The information on by-product flows like bauxite residues were taken from World Aluminium (2020). The BAT document from the European Commission 22 was used for a simplified reference process for secondary aluminium production.
For cement production, the data for global average production and its inputs was taken from GNR (2017) and two further processes with a higher intake of cementitious materials (fly ash and slags) were added based on the processes described in The mass flows for paper production were taken from the BAT 28 , the life-cycle inventory in Corcelli et al. 26 and for paper recycling from Christensen and Damgaard. 27 The GHG emissions factors were taken from the BAT documents and for pulp production from the data provided in the Ecoinvent database. 29 S14

Cement production
The production of cement consists of the processes for raw meal, clinker and cement production.

Aluminium production
For aluminium production, the data provided by World Aluminium 33 and World Aluminium 34 was used for the major mass flows. The production data was extended with the input-balances provided by European Commission 22 and case studies of aluminium production by Balomenos et al. 35

Paper production
For paper production, a typical tissue mill in the European Union 28 was used as a reference for the energy and mass flows.

Energy processes
Various processes for electricity generation, heat generation and heat exchange are used in the model. The following sections summarise the key processes and provide references for the values used for the stylised routes.

Electricity generation
There are four major processes for generating electricity. For the electricity generation in the reference scenario, the global average emissions of electricity production (0.475 kg/kWh) is used 36 .

Figure S 8: Process for average global electricity generation
The GHG emissions for the processes were taken from 37 for coal, gas and biomass combustion and summarised in Table S Coal boiler 89% The average efficiency of 89% was chosen for steam generation from coal.

80%
The average efficiency of 80% was chosen for steam generation from biomass. It is assumed that steam generation from black liquor (BL) has a similar efficiency.

Paoli and Cullen (2020) 41
The steam requirement was usually reported in MJ/kg. For the processes, for which a direct steam flow was required, the steam flows were calculated. The data for the steam heat values was used from steam tables in Rogers and Mayhew. 42 In the reference scenario without industrial symbiosis, current combustion processes are used for preheating substance flows. In the scenario of industrial symbiosis, further options for heat exchange between processes are added. The preheaters either directly heat up cold substance flows (e.g. through electricity or fuel) or indirectly heat processes through heat transfer from another hot fluid that is cooled down in the heat exchanger. For the heat exchangers in the model, a heat transfer efficiency 76% was used for the heat exchanger networks. 43

Options for industrial symbiosis
How can industrial by-products be used as an input to another process? The processes that prepare and use those by-products are referred to as "processes of industrial symbiosis" in this model. There are several options for industrial symbiosis, which have been applied at different scales across different industrial processes. A literature review of major case studies of industrial symbiosis was used to obtain a database of by-product exchanges and to classify the symbiotic options for the major bulk material production processes.

Reference scenario Scenario A Scenario B Material production
Global material production in 2017.

Main processes
Production processes for major bulk materials. Auxiliary processes Same auxiliary processes for providing the external inputs. Minimum production Full combustion of black liquor (BL) and blast furnace (BF) gas within the production system.

Use of secondary raw materials and byproducts
Based on current (2017) global average intake, e.g. in cement production. Only a few by-product utilisation processes (representing current use).
No constraints on the maximum intake of cementitious materials (given supply constraints). More byproduct utilisation processes are operating.
No constraints on the maximum intake of secondary materials. All byproduct utilisation processes are operating.

Heat exchange and utilisation
Processes for heat recovery are not operating.
Processes for heat recovery are not operating.
All symbiosis processes and options for heat exchange and recovery (ORC turbines) are operating.
The symbiosis processes include processes for heat recuperation as well as material utilisation for substituting another input. In order to classify the processes, four different categories were used. These categories reflect the degree to which the technologies for by-product exchanges have been developed or applied. The categories are the following: 1) Commercially applied, processes with high technology readiness level (TRL). 2) High TRL, applied in several but not all countries. 3) Pilot projects and case studies of industrial symbiosis. 4) Not currently deployed (e.g. economic constraints) or no information on application available, literature suggests a potential for symbiosis.
The processes in category 1 were used in the reference scenario. The processes in category 2 are included in the symbiosis scenario A and the options in categories 3 and 4 in scenario B.

Waste heat recovery
In the analysis of a global symbiosis, the urban sector is included via heat recovery e.g. in Organic Rankine Cycle turbines in scenario B. The detailed inclusion of the urban sector, i.e. district heating networks was not included due to several reasons: there are practical limitations of implementing these technologies for remote production sites and the overall utilisation potential is dependent on available infrastructure, such as district heating systems which would need to be included in the emissions accounting. Some practical limitations are the following: • Timing: The waste heat availability ("generation" does not necessarily match with the heat demand by households. This is especially important for batch processes, e.g. electric arc furnaces for which steam accumulators are required. 45 For periods without heat demand in summer, additional ORC turbines are needed 46 to capture the waste heat. • Distance: The efficiency of heat transport and provision limits the potential demand to areas around industrial clusters: The recommended maximum distance of the pipe infrastructure between heat source and user is between 10 kilometres 47 and 15 kilometres 48 in order to avoid major heat transport losses. Hence, there are three categories of residential buildings which can use the waste heat: (1) new buildings, (2) buildings with decentralised heat generation that can be changed to district heating and (3) buildings already connected to the DH grid. Older buildings, especially Victorian houses, will most likely not profit from the DH since they are not connected to the DH network. Overall, the distance of the generation to the district heating network reduces the overall potential though some first studies estimate that the distance can be increased to a few dozen kilometres. 49 The utilisation of waste heat, however, could assessed in a more detailed geographical analysis of potentials of symbiosis. generation during production stops (planned and unplanned) as well as provision of heat in intermitting processes (e.g. batch-processes like the EAF) pose further challenges to the operation of DH networks. If planned carefully, the DH networks could be used as a thermal heat storage to balance intermittent renewable electricity using heat accumulators 54 . Current design of DH networks, however, allows for only limited flexibility. 55 Hence, gas-or coal-fired power plants would be needed as a back-up capacity to the heat networks. These might then have higher carbon footprints per unit of heat delivered (compared to decentralised heating), reducing the overall efficiency gains through DH networks.
For the recovery of waste heat from hot flue gas streams, Organic Ranking Cycle (ORC) turbine processes were added. These generate electricity from hot flue gases through a primary and secondary steam cycle using various fluids. The typical efficiency the ORC turbine of 15% is used. 56 Figure S 9 illustrates a representative and a simplified process design of conversion of hot flue gases to cold flue gases and electricity. The ORC turbines in the model follow the simplified process design.

Figure S 9: Organic Rankine Cycle (ORC) turbine used in the model
For some heat recovery processes, data on available processes for heat recovery was available. These reported implementation of heat recovery options in the industrial processes were used for several flows. The processes are summarised in Table S 25 and added to the production system as "symbiosis processes" and operate in scenario B.

Solid waste as a fuel
The use of solid waste was included in the model as a fuel for cement production using current global average co-incineration of alternative fuels. An example case study using municipal solid waste as a fuel is provided by Hashimoto et al. 57 In that, they use a LCA-based substitution analysis to assess the effect of a fuel switch from coal to municipal solid waste, which have a slightly lower carbon footprint, in four scenarios of symbiosis. The carbon footprint of the fuel use is reduced from 2.58 tCO2/t (coal) to 2.44 tCO2/t (MSW), leading to an overall emissions reduction of around 44ktCO2 per year (10% of the combustion-related emissions).
If biomass fuels or alternative fuels are combusted instead of fossil fuels, the overall emissions might remain constant (or increase) while the fossil-based emissions are reduced. Figure 18 in the report by Hinkel et al. 58 visualises emissions factors and typical biomass content of different alternative fuels and contrasts them with fossil fuels currently used in cement production. The biomass content of typical fuels such as animal meal and sewage fuel is up to 100% with an emission factor of up to 110 kgCO2/GJ) The emissions factor for fossil fuels is significantly lower (55-90 kgCO2/GJ), though these emissions are solely fossil-based. If the emissions accounting from the WBCSD 59 is used, the biomass-based emissions are not allocated to cement production. Additional through MSW combustion were not attributed to the co-incineration since the model does not differentiate between biogenic and fossilbased CO2 emissions.
The analysis in this paper, however, includes all carbon dioxide emissions regardless of their origin (biomass/fossil-based). Hence, the emissions from biomass-based waste are included as normal GHG emission and additional GHG emissions reduction credits were not attributed to the processes. The theoretical potential of GHG emissions reduction reported in Hashimoto et al. 57 considers the biomassbased emissions as emissions savings since they use a LCA-based emissions factor. Hence, they estimate a larger potential for savings due to the fuel switch to MSW.

Summary of the processes in the model
The processes used in the scenarios are summarised in this section. Table S 26 summarises the processes of the production system. For each process, a short description is provided. Produces raw meal for clinker production Clinker production ce_clinker_production Produces clinker for cement production Clinker production (T2) ce_clinker_production_t2 Produces clinker for cement production Cement Production (Global Average) ce_cement_production_global_avg Global average cement production based on GNR data Cement Production (T1) ce_cement_production_s30 Cement production process with byproduct use 1 Cement Production (T2) ce_cement_production_s31 Cement production process with byproduct use 2 Cement Production (T3) ce_cement_production_s32 Cement production process with byproduct use 3 Preparation of clinker for cement (heating) ce_clinker_preparation_for_cement Overall cement production Overall cement production ce_total_cement_production Preparesclinker for use in cement production Electricity generation (cement) Provides electricity for aluminium production Mechanical pulping pp_pulp_mech_chem Mechanical and chemical pulping process for paper production Chemical pulping pp_paper_recycling Paper recycling processes Heat provision for paper production pp_heat_paper_recycling Provides heat/steam for paper production Papermaking production pp_paper_primary_route_making Heat provision for paper recycling Global paper demand pp_paper_demand Global paper demand Primary paper production pp_paper_primary Overall primary paper production via primary route Paper recovery pp_paper_secondary Uses pulp for producing paper Electricity generation (paper)

Model implementation
This sections provides a description of the model and the implementation in python. Figure S 10 summarises the input files and code used for the industrial symbiosis model. The input files are the material demand, symbiosis scenarios and the production coefficient matrix.

Figure S 10: Overview of the model in python
For the model implementation in python (version 3.9.6), the mathematical programming package scipy (version 1.1.0) is used.

Sensitivity analysis
The goal of the uncertainty and sensitivity analysis is to assess the robustness of the symbiosis model with regard to changes in the input parameters. The global symbiosis model has several input parameters which influence the overall system's GHG emissions. The material demand, the production-coefficients, the process-level GHG emissions and the supply constraints are externally defined. Whereas the production coefficient matrix contains the constant production coefficients (recipes), the other three main parameters can be varied to assess the sensitivity and stability of the model results.
o The global material demand ( "), o The process-specific GHG emissions ( ! ), o The supply constraints, i.e. process rates of the external processes that provide fly ash or secondary resources ( ! ), The sensitivity analysis was conducted (1) for the process-specific GHG emissions and (2) for the constraints on the external supply of fly ash.

Changes in the process-specific GHG emissions
The process-specific GHG emissions ( ! ) include the direct process-level GHG emissions of every production process included in the model. The indirect emissions are covered through the energy inputs (e.g. electricity) of the processes with a process-specific GHG emission factor. The coefficients for the direct emissions for the major bulk material processes were obtained from the literature review. The values ( ! ) were varied by ±10% and ±20% for every process. The findings are summarised in Figure   The relative changes of the GHG emissions in the reference scenario are consistent with an increase or decrease of the specific emissions factors. Further research could include a more detailed range and uncertainty reporting of the process-level GHG emissions to explicitly include uncertainty in the GHG emission factors as well as the production coefficients.

Fly ash supply
The emissions reductions in cement production mainly stem from the utilisation of fly ash as a cementitious material (with up to 35% use of fly ash). The cement sector emissions reductions (17%) in scenario A are mainly driven by the utilisation of fly ash. Figure S 13 visualises the impact of increasing fly ash supply on the sectoral GHG emissions in cement production for different levels of fly ash supply.

Figure S 13: Sectoral GHG emissions in cement production in scenario A
Overall, the relative changes of overall emissions due to changes of the process-level emission factors lead to a similar increases and reductions in system GHG emissions across all scenarios. The changes in the availability of the secondary materials lead to (small) changes in the overall emissions in the reference scenario. Further research could include a more detailed assessment of changes and uncertainty of the production coefficient matrix.